Generally, a thermosensitive recording medium, wherein recording images are obtained by the use of color developing reaction by heat between a colorless or pale colored electron-donating leuco dye (henceforth referred to as “dye”) and an electron-accepting color development agent (henceforth referred to as “color development agent”), has been widely used in the fields of facsimile, computer, various measuring instruments and the like because of the advantages in very clear color development, noiseless recording, compact and relatively inexpensive apparatus, and easy maintenance. Recently, the application of the recording medium has been rapidly expanded to output media of various printers and plotters such as handy terminals for outdoor measurement, delivery slips, in addition to labels and tickets. Therefore, the quality of the thermosensitive recording medium is required to have better developing sensitivity and better printing run-ability (prevention of sticking and head debris) than before, and furthermore to have aptitude to general printability (surface strength, ink receptivity, etc.) for offset printing. Moreover, such labels, tickets, handy terminal papers and delivery slips are used in the open air, then the recording medium should be tolerant for water, damp, sun light and high temperature to be used under sever environmental conditions. Therefore, it is required to have an advanced water resistance and a resistance for color development during storage.
To add the properties of an offset printing method, which is a different printing method from a thermosensitive recording method, to a thermosensitive recording medium, several thermosensitive recording media have been developed, such as a thermosensitive recording medium containing fluorine-based resin in the thermosensitive recording layer (Reference 1), a thermosensitive recording medium containing pigments having a certain mean particle diameter, specific surface area, and fine pore volume in a thermosensitive recording layer (Reference 2), and a thermosensitive recording medium containing pigments with certain oil absorption and emulsion of hydrophobic resin in a thermosensitive recording layer (Reference 3).
Still furthermore, a technique of adding acrylate resins in a thermosensitive recording layer has been developed to improve water resistance, and a technique of using simultaneously acrylate polymer, acrylate emulsion resins and colloidal silica in a thermosensitive recording layer has been developed to improve water resistance and head debris resistance (References 4 and 5).
Additionally, an alkylketene dimer used in the present invention has been used as a neutral sizing agent and it is well known that an alkylketene dimer produces a sizing effect by binding to a cellulose at a paper making process (Reference 6 etc.).    Reference 1: Japanese Patent Application Public Disclosure H09-142018    Reference 2: Japanese Patent Application Public Disclosure 2005-199554    Reference 3: Japanese Patent Application Public Disclosure H10-250232    Reference 4: Japanese Patent Application Public Disclosure H09-207435    Reference 5: Japanese Patent Application Public Disclosure H07-266711    Reference 6: Japanese Patent No. 3755483